Communication, radio communication among others, has become rapidly popular in recent years, so that a higher speed and a greater capacity of communication are required in communication systems. A band having unused radio resource can achieve a higher speed by broadening the band. However, a band having little unused radio resource must employ a technique of using a higher modulation level for increasing its communication speed. This technique upgrades the modulation level at the modulation without changing the occupying bandwidth for modulating signals. This technique can be also applied to the case where a user tries to increase a communication speed within the range of a bandwidth assigned. The foregoing technique is one of effective measures for increasing a communication capacity
In the case of mobile communication, a move of a radio station changes conditions of communication channels. In such an environment, a technique of adaptive modulation has been discussed as one of effective measures to enlarge a communication capacity, and yet, this technique can improve the quality of communication. The adaptive modulation technique switches a modulation method to/from another method adaptively in response to conditions of a transmission path. (reference: “Mobile Communication” written by Mr. Shuichi Sasaoka, chapter 5, published from Ohm Co. Ltd. May 25, 1998)
A conventional communication system employing the adaptive modulation technique is described with reference to FIG. 19, and the description refers to not only the structure but also the operation of the system. Between communication devices 001 and 002, a bi-directional communication link and frequency division duplex (FDD) are used. The communication link from device 001 to device 002 is called a forward link, and the link from device 002 to device 001 is called a return link.
In communication device 001, adaptive modulator 0011 modulates transmission data stream 001 by a given modulation method, and transmission processor 0012 converts and amplifies the frequency of stream 001 in a given manner, before transmitting data stream 001. An example of the modulation method used in adaptive modulator 0011 is Quadrature Phase-Shift Keying (QPSK) modulation or hexadecimal Quadrature Amplitude Modulation (QAM). An operation, where those two methods are selectively switched, is described hereinafter.
In communication device 002, reception processor 0021 receives a signal radio-transmitted from device 001, then amplifies the signal and converts the frequency of the signal in a given manner. Adaptively modulated signal demodulator 0022 demodulates the signal by a demodulation method corresponding to the modulation method used in adaptive modulator 0011, thereby obtaining a reception data stream. On he other hand, reception quality estimator 0023 uses the reception signal received by device 002 to estimate communication quality at the forward link.
The communication quality is obtained as reception quality information resulting from the calculation of a CNR value (Carrier Power vs. Noise Power). Reception quality information transmitter 0024 transmits the quality information to communication device 001 via the return link. In device 001, reception quality information receiver 0013 receives the quality information, and modulation method controller 0014 selects an appropriate modulation method to be used at the forward link based on the quality information, and controls adaptive modulator 0011 to use the modulation method thus selected.
Communication device 001 is required to notify device 002 of the information about the modulation method selected. This is done by, e.g. placing a symbol for identifying the information about the modulation method at a given place in a transmission burst, and device 002 recognizes this symbol at the given place, thereby identifying the modulation method to be used.
The structure and operation discussed above allows communication device 001 to monitor the communication quality at the forward link and select a modulation method having enough error tolerance although having a lower modulation level in a condition of poor communication quality. On the contrary, in a condition of excellent communication quality, device 001 selects a modulation method having a higher modulation level for increasing transmission capacity although having less error tolerance. As a result, in the case of poor communication quality, the transmission capacity is kept at a lower level and yet communication reliability is maintained for retaining the communication link. When the communication quality becomes better, the transmission capacity can be increased.
The foregoing conventional structure is obliged to switch a modulation method at least by a unit of a burst, i.e. on a burst basis, or by a unit of plural bursts, so that a probability of reception errors occurring in a burst differs in a great amount depending on QPSK modulation or hexadecimal QAM (hereinafter simply referred to as 16QAM.)
To be more specific, there can be a condition where no error happens at all in the communication by QPSK modulation, but errors can happen in bursts in the communication by 16QAM modulation. It can also sometimes happen that estimation of communication quality by 16QAM modulation becomes difficult while the reception device receives signals modulated by QPSK modulation.
In an actual communication environment, communication quality differs depending on places in a communication burst due to various factors. For instance, in a reception device, which uses a preamble inserted in a burst for time-sync tracking or equalization, reception quality is sometimes relatively excellent at a symbol frame closely following the preamble, and the reception quality becomes relatively poor at a symbol away from the preamble timewise.
In such a communication environment, although a burst has some frames where communication by 16QAM can be done, the conventional adaptive modulation method is obliged to select a modulation method only by a unit of a burst, so that QPSK modulation is resultantly selected. Thus the conventional method can obtain only a little advantage of increasing the communication capacity.